In recent years, a variety of three-dimensional (3D) integration and packaging techniques have been examined. The main considerations behind the use of 3D integration are: 1) minimization of the wire length, 2) incorporation of new back-end-of-the-line (BEOL) processes that are currently limited by conventional planar technology, and 3) implementation of related design flexibility. Items 1-3 mentioned above would allow significantly reduced interconnect delay as well as a complex system integration to increase both performance and functionality.
Approaches to 3D integration at either the chip or wafer level have been described in the prior art. For example, wafer level bonding can be achieved via an assembly approach. In such a method, layers are transferred one by one, on top of each other, and attached by a bonding process. The prior art layer transfer process is realized using carrier wafers, most often a glass substrate.
In such a scheme, the glass substrate is attached to the structure by an adhesive bonding process and released after the layer transfer is completed. One of the methods to release glass is based on laser ablation, which entails irradiating the glass/adhesive interface through the back surface of the glass substrate. In order to accomplish the ablation process, polyimide materials are typically used as a sacrificial adhesive layer in prior art 3D integration schemes. The polyimide sacrificial adhesive layers are deposited on top of the layer that will be subsequently transferred. During ablation, the deposited energy is contained within a shallow (submicron) surface layer for an approximate 50 ns duration of the excimer laser pulse due to the polyimides strong absorption properties of ultraviolet laser radiation and poor thermal conductivity. When the absorbed energy density exceeds a certain threshold value, a surface layer having a thickness of less than 1 μis photo-ablated and the laser separation of the glass carrier substrate is realized. The laser ablation process using polyimides has been reported and a comprehensive summary has been provided by Srinivasan, et al., “Ultraviolet Laser Ablation of Organic Polymers”, Chem. Rev. 990, 1303-1316 (1989).
The assembly approach in which laser ablation is used is only one of the examples in which the polyimide material is used in a 3D integration scheme. In general, in 3D structures, the polyimide layer is deposited on an already processed and tested device layer terminated with at least one Cu-based wiring layer. When a polyamic acid (PAA) solution, which is the precursor for the formation of polyimide films, is spin applied to the Cu surface and subsequently cured at a temperature between 350°-400° C., Cu reacts with the polyamic acid during the curing step to form salts which diffuse into the polyimide layer to form copper oxide precipitates. This is disclosed, for example, in Kim, et al., “Adhesion and Interface Investigation of Polyimides on Metals”, J. Adhesion Sci. Technol., Vol. 2, No. 2, pp. 95-105 (1988). As demonstrated by Kowalczyk, et al., “Polyimide in Copper: The Role of Solvent in the Formation of Copper Precipitates”, Appl. Phys. Lett., Vol. 52, No. 5, pp. 375-376, (1988), the polyimide precursor solvent, n-methyl pyrrolidone (NMP), provides mobility for the aggregation of Cu precipitates.
This situation is worsened when photosensitive polyimides are used since reacted Cu leaves a residue upon development, which is very difficult to clean; see, in this regard, Perfecto, et al. “Evaluation of Cu Capping Alternatives for Polyimide-Cu MCM-D”, ECT. '01 (2001). In the case of a preimidized polyimide, Cu diffusion has been observed and documented in U.S. Pat. No. 5,081,005. Over the years, the copper-polyimide interface has been well studied. Copper-polyimide technology has been successfully used in the form of multi-level thin film structures for over two decades now. It has been primarily developed for use in the cost/performance SCM's and high end MCM's applications; see, for example, Prasad, et al., “Multilevel Thin Film Applications and Processes for High and Systems”, IEEE Transactions and Components, Packaging, and Manufacturing Technology-Part B, Vol. 17, No. 1, pp. 38-49 (1994).
In these applications, to prevent copper diffusion into the polyimide, various metal capping layers have been used. Illustrative examples of prior art polyimide capping layers include, for example, Cr, Pt, Pd, Ti, Co(P), and chromate treatment; see, in this regard Matienzo, et al., “Adhesion of Metal to Polyimides, in Polyimides: fundamentals and applications”, K. K. Ghosh and K. L. Mittal Eds., Marcel Dekker, NY, N.Y. (1996); Shih, et al. “Cu passivation: a method of inhibiting copper-polyamic acid interactions”, Appl. Phys. Lett., Vol. 59, No. 12, pp. 1424-1426 (1991); Ohuchi, et al., “Summary Abstract: Ti as a diffusion barrier between Cu and polyimide”, J. Vac. Sci. Technol. A, Vol. 6, No. 3, pp. 1004-1006 (1988); O'Sullivan, et al., “Electrolessly deposited diffusion barriers for microelectronics”, IBM J. Res. Develop., Vol. 42, No. 5, pp. 607-619 (1998).
Also, baseline requirements for a capping layer in the Cu-polyimide system used for various packaging structures have been established. Namely, any Cu passivation metal should be chemically inert and insoluble in PAA; and the passivation metal should be a good diffusion barrier against Cu out diffusion at temperatures less than 100° C. when the solvent NMP is present (above this temperature the Cu transports into the polyimide via solid-state-diffusion). Moreover, the passivation metal should not diffuse into Cu to cause resistivity increase.
In addition to copper diffusion barrier properties, metal caps were found to enhance adhesion between Cu and a polyimide. The shortcoming of this Cu/metal cap/polyimide is based on the processing limitation, for example, when the metal wiring is defined by the subtractive etching of a Cr/Cu/Cr sputtered film, Cr protection only occurs on the top of the wiring. Similar problems take place when a metal is deposited by a lift-off process. Hence, this solution has been limited to pattern electroplated films, where Co or chromate treatments have been shown to successfully encapsulate the Cu wiring.
However, in case of 3D integration applications, the concern about metal capping layers is based on compatibility of these materials with various heterogeneous structures involved in future 3D integration schemes. The capping could be introduced as a continuous layer across the whole wafer. In this case, after the layer transfer and ablation of the glass substrate is completed, this layer would be exposed to the removal of the polyimide (the removal step is not present in the aforementioned packaging applications). Wet and dry methods have been used to remove polyimides, but oxygen-plasma based removal has been proven most effective, and it is also is a well understood process.
Therefore, in case of 3D structures, requirement of good Cu-diffusion barrier (specially against activated oxygen in a plasma etching environment) is additionally mandated of the capping layer. Since titanium is prone to oxidation in an oxygen-plasma, it cannot be considered as a candidate for a capping layer. Even if other capping metal candidates are stable in the oxygen-plasma environment, once the polyimide stripping process has been completed, the additional step of removing the sacrificial capping layer would have to be implemented in order to provide electrical separation between Cu wires. This removal process needs to be CMOS compatible, and preserve the structural, mechanical and electrical stability of the underlying patterned structures. Selective etching of such capping metals without degrading (etching or damaging) the underlying copper wires makes the choice of such a metal cap layer even more difficult. Taking all these restrictions into consideration, the metal capping-sacrificial coating of a full wafer is not likely to be feasible from the manufacturing point of view.
The metal capping in the form of a selective cap, such as electroless Co on the top of Cu structures, could be implemented in a 3D integration scheme. However, application of such a cap will be limited, as 3D structures may implement various heterogeneous materials and their compatibility with Co, or other relevant selective metal caps would have to be established.
The organic copper-capping technology for the Cu-polyimide system was also developed for thin film packaging. It has been shown that a thin organic coating, such as poly(arylene ether benzimidazole) (PAEBI), silane-modified polyvinylimidazole, or polybenzimidazole, can be applied directly to a wiring layer for enhancing adhesion to both the copper wiring and the polymer dielectric surface. These materials provide 100% protection for copper wiring, eliminating the need for metal capping, but at the expense of adding a thermal treatment step prior to the coating of the polyimide. This is described, for example, in Lee, et al., “Adhesion of poly (arylene ether benzimidazole) to copper and polyimides”, J. Adhesion Sci. Technol., Vol. 10, No. 9, pp. 807-821 (1996); and Ishida, et al., “Modified Imidazoles: degradation inhibitors and adhesion promoters for polyimide films on copper substrates”, J. Adhesion, Vol. 36, pp. 177-191 (1991). Such predominantly organic caps will be attacked by oxygen plasma exposure and will not protect the copper wires during the post ablation cleaning step of plasma ashing.
Organic caps that do not require additional thermal treatments have been evaluated by Perfecto, et al., “Evaluation of Cu capping alternatives for cu-Cu MCM-D, ECTC'01(2001).
Two approaches were investigated in the Perfecto, et al. paper: 1) re-formulation of the PAA with an additive which will reduce the Cu diffusion and/or prevent Cu from complexing with the PAA, and 2) spun dry precoat of a Cu surface with an organic solution that reacts with Cu reducing the availability of Cu for diffusion. In the first method, 1% tetrazole in a polyimide solution, and 5% benzotriazole (BTA) in a polyimide solution were evaluated, while in the second method an amino silane, namely, 3-aminopropyl-trimethoxy silane diluted to 1% in deionized water, as well as BTA diluted to 1% NMP were studied. All systems showed degraded performance when compared to the simplest and most robust process of coating copper with 3-aminopropyl-trimethoxy silane. A layer of 3-aminopropyl-trimethoxy silane exhibited superior performance as an adhesion promoter in the Cu-polyimide system, and as a Cu-diffusion limiting layer, and its use as a capping layer in package-related applications has been described in U.S. Pat. Nos. 5,081,005 and 5,194,928.
However, a coating of 3-aminopropyl-trimethoxy silane (usually a few monolayers) is not stable in the plasma-environment, and hence it cannot serve as an oxygen-diffusion barrier. Therefore, its use as a capping layer in the 3D integration applications is limited to schemes when no oxygen-plasma processes are involved. However, other characteristics of 3-aminopropyl-trimethoxy silane, such as its ability to promote interfacial strength in both polyimide/silicon dioxide and silicon/silicon nitride laminates, make this system a great candidate in the scheme for capping layer discussed below.
In view of the above, there is a need for providing an improved capping layer which provides adhesion as well as protection to underlying layers such as metal-based semiconductor elements.